1. Field
The invention is in the field of detection methods and apparatus based on Schlieren optics.
2. State of the Art
It has been known for some time that a refractive index gradient such as produced by a concentration gradient in a fluid such as a gas, liquid, or supercritical fluid, will cause deflection of light passed through the gradient. The optical method of observing and measuring the deflection of light caused by refractive index gradient fields is generally referred to as Schlieren optics. In the past, Schlieren images resulting from light deflections have been recorded on photographic plates and the plates then analyzed for light intensity distribution using densitometers. Recently, evaluation of the photographic images has been done by computer. These methods are useful in studying plasmas where very complicated toroidal and parabolic shapes are generated.
U.S. Pat. No. 4,547,071 discloses a sensor for measuring density gradients in a nonhomogeneous fluid sample using Schlieren optics. In such sensor, a laser light beam is directed through a sample chamber and is moved along said chamber. A quadrant light position sensor located on the opposite side of the chamber detects the deflection of the laser light beam as it is moved through the sample. The amount of deflection indicates the density gradient at any point in the sample. Rather than moving the laser beam along the sample chamber, the beam can be held constant and the sample moved within the chamber.
A current development in the field of high performance liquid chromatography (HPLC) is the open tubular capillary column which provides ultra high efficiency separation of sample components. This method can utilize very small sample volumes. Similarly, current capillary zone electrophoresis equipment can be used with extremely small volume samples. My U.S. Pat. No. 4,784,494, shows a detector that can be used to detect concentration and thermal gradients in very small samples. That detector utilizes a light source to generate a probe beam of light that is passed through the sample having the gradient to be detected or measured and the deflection of the probe beam is measured on a beam position detector. Various light sources may be used to generate the probe beam such as a laser or light emitting diode (LED).
With equipment that detects or measures concentration or other gradients present in a sample, the sensitivity of such equipment is limited by the extent of the gradient present in the sample. The concentration gradients associated with solutes eluting from typical HPLC columns are generally small due to significant band broading during the separation process with the result that sensitivity of the gradient detection using currently known detection equipment is poor. Thus, the single probe light beam detection equipment shown in my U.S. Pat. No. 4,784,494, or the dual probe light beam equipment shown in my parent application, when used in the embodiments specifically illustrated and described, are subject to poor sensitivity when the actual concentration gradients to be detected are small. Increased sensitivity in detecting small gradients would be desirable.